Prevent Unicorn crashing in development on macOS

What does this MR do?

Adds a macOS dlopen call during Unicorn startup to prevent crashes on High Sierra, for development only.

Based on https://bugs.ruby-lang.org/issues/14009.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Because Unicorn can crash on macOS High Sierra with an error related to forking. The fix here is more 'surgical' than the previous workaround of setting BJC_DISABLE_INITIALIZE_FORK_SAFETY=YES in the environment of Unicorn. Plus, it's automatic.

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Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

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